A transit-time ultrasonic gas flowmeter for high-rangeability requirements, such as those encountered in flare-gas flow-metering, is presented. The concept of ray rescue angle for the orientation of the ultrasonic transducers in single-beam transit-time ultrasonic flowmeters is introduced to overcome the problem of ultrasonic beam drift in high-velocity flows. To overcome problems associated with noise at high velocities, a chirp signal is used. To preserve the accuracy of the meter at low velocities near zero flow, a combination of chirp and continuous-wave signals is used to interrogate the flow. Overall system performance is presented, based on results from extensive wind-tunnel tests.

The signal processing used in an ultrasonic high-rangeability gas flow meter using times of flight is presented. The flow meter under discussion uses a combination of continuous wave and chirp signals to measure at low flow velocities, below 20 m/s, and chirp signals alone to measure high flow velocities, above 20 m/s. Because of the need for a pulse compression technique in the signal waveform design the technique of pulse compression and the choice of signal waveforms are discussed. The advantages and disadvantages of amplitude weighting vis-a-vis frequency domain manipulations of the waveforms are also discussed. To eliminate spurious times of flight, a special filtering technique is used, based on assessing the gradient of ascendingly ordered time series of time-of-flight measurements. A summary of user experience with high-rangeability gas flow meters in use on different offshore platforms and in refineries is given. Long-term tests that examined the accuracy of the high-rangeability flow meter are also described.

The importance of ray rescue angle orientation of ultrasonic transducers for high-rangeability ultrasonic llowmetering is known from earlier publications. The adaptive signal processing based on pulse compression in conjunction with the ray rescue angle orientation of the transducers is another important feature for achieving high-rangeability ultrasonic gas flowmetering. The technique of achieving the necessary broad bandwidth of the ultrasonic transducer, which has to be excited within a low lrequency band due to the absorption of ultrasound in gas increasing with lrequency is described. A signal processing system, catering to both very low and high times-ol-flight and their differences, is also presented. The usage of a special filtering technique based on assessing the gradient of the time series, in ascending order, of times-of-flight to eliminate spurious times-offlight observed in actual internogations of the flow is also presented. After presenting this technique, the paper preserlts a summary of user experience of high-rangeability gas flowmeters in use for over two years on different platlorms and in refineries. Hereby, the sensor performance and the long term tests that looked into the accuracy of the high-rangeability flowmeter are described. Results lrom on-and offshore installations of the ultrasonic flowmeters monitoring flare gas are also presented. TIMES-OF-FLIGHT IN HIGH-RANGEABILITY GAS FLOWMETERINGThe acoustic paths in the gas and along the geodesic line in the wall of a flow pipe are shown in Fig. 1. Two ultrasonic transducers are shown communicating with each other. The normalised timesof-llight for acoustic paths in the gas and the normalised propagation time in the wall of the flow pipe are shown in Fig. 2, where t,, and t, , are downstream and upstream times-of-flight respectively, 1, is the propagation time along the geodesic line in the pipe wall between the transducers and t, is the time-of-flight

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